A memory density of semiconductor memory devices as represented by a DRAM (Dynamic Random Access Memory) increases year by year based on progress of microfabrication. However, along the progress of microfabrication, the number of defective memory cells included in one chip also increases in the actual situation. These defective memory cells are usually replaced by redundant memory cells, thereby relieving defective addresses.
In general, defective addresses are stored in a program circuit including plural fuse elements. When an access to a defective address is requested, the program circuit detects this access, and performs so that the alternate access is made to a redundant memory cell instead of this defective memory cell. As a configuration of the program circuit, there is known a system that allocates a pair of (that is, two) fuse elements to each bit constituting an address to be stored, and that stores a desired address by disconnecting one of the pair of fuse elements, as described in Japanese Patent Application Laid-open No. H9-69299.
There is also known a system that allocates one fuse element to each bit constituting an address to be stored, as described in Japanese Patent Application Laid-open No. H6-119796. According to this system, one bit can be stored depending on whether one fuse element is to be disconnected. Therefore, the number of fuse elements can be substantially decreased.
There are broadly two methods of disconnecting a fuse element: a method of disconnecting a fuse element by fusing the fuse element using a large current (see Japanese Patent Application Laid-open Nos. 2005-136060 and 2003-501835); and a method of disconnecting a fuse element by breaking the fuse element by irradiating a laser beam (see Japanese Patent Application Laid-open Nos. H7-74254 and H9-36234). The former method has advantages in that the method requires no expensive device such as a laser trimmer, and can easily self-diagnose whether a fuse element is correctly disconnected. However, in using this method, a fuse disconnecting circuit and a diagnosis circuit need to be incorporated in a semiconductor in advance. This has a disadvantage of increasing a chip area.
On the other hand, the method of breaking a fuse element by irradiating a laser beam does not require a fuse disconnecting circuit to be incorporated in a semiconductor device in advance. Therefore, a chip area can be decreased by this method.
FIG. 11 is a schematic cross-sectional view for explaining a method of disconnecting a conventional fuse element by irradiating a laser beam.
A fuse element 10 shown in FIG. 11 is located on a higher layer than lower-layer wirings 11, and both ends of the fuse element 10 are connected to the lower-layer wirings 11 via through-hole electrodes 12. To disconnect the fuse element 10 having this configuration, a laser beam L is irradiated to the fuse element 10 from above the fuse element 10. In this case, the laser beam L is converged to include the fuse element 10 within a focal depth of the laser beam L, thereby focusing heat energy onto the fuse element 10 to disconnect the fuse element 10.
Before the fuse element 10 is disconnected, the two lower-layer wirings 11 shown in FIG. 11 are electrically connected to each other. When the fuse element 10 is disconnected, the two lower-layer wirings 11 are electrically insulated. In this way, a connection state of the lower-layer wirings 11 can be irreversibly changed by using the fuse element 10. With this arrangement, a defective address can be stored permanently.
To store a defective address and the like, many fuse elements 10 need to be laid out. Therefore, at the time of disconnecting a predetermined fuse element by irradiating a laser beam, influence given to adjacent fuse elements and to their peripheries need to be considered.
FIG. 12 and FIG. 13 are explanatory views of the influence applied by the laser beam to other fuse elements and to their peripheries. FIG. 12 is a top plan view, and FIG. 13 is a schematic cross-sectional view along a line D-D shown in FIG. 12.
As shown in FIG. 12 and FIG. 13, at the time of disconnecting a predetermined fuse element 10a by the laser beam L, a beam spot of the laser beam L is narrowed by the fuse element 10a to have a minimum diameter D1. However, not all the energy of the laser beam L is absorbed by the fuse element 10a, and a part of the laser beam L is leaked out to a semiconductor substrate outside the fuse element 10a. Because the beam spot of the laser beam L is narrowed to a minimum at the fuse element 10a, the beam spot of the laser beam L becomes larger than D1 at the semiconductor substrate side than at the fuse element 10a. 
Therefore, when a planar distance between adjacent fuse elements 10a and 10b is short, the laser beam L is irradiated to a peripheral circuit of the fuse element 10b, as shown in FIG. 12 and FIG. 13. In the example shown in FIG. 12 and FIG. 13, when a layout pitch of the fuse elements 10a and 10b is P, the beam spot of the laser beam L extends to about 2P (=D2). As a result, the laser beam L is irradiated to the lower-layer wiring 11b and the thorough-hole electrode 12b. 
In the state that the beam spot is spread, while the energy density of the laser beam L is low, the adjacent lower-layer wiring 11b and the thorough-hole electrode 12b have a risk of being greatly damaged depending on conditions. For example, when the through-hole electrode 12b pierces through a part of an insulation layer having a high light-absorbing rate like a silicon nitride film, the through-hole electrode 12b has a risk of being broken at this part. Therefore, when the layout pitch P of the fuse element is narrowed, this has a problem of a reduction in the reliability of the semiconductor device. To prevent this problem, the layout pitch P of the fuse elements is set large. However, in this case, the number of fuse elements that can be laid out per unit area decreases.
As described above, the method of breaking fuse elements by irradiating a laser beam has the problem in that reliability decreases when the layout pitch P of fuse elements is made small, and that the degree of location becomes low when the layout pitch P of fuse elements is increased. Even when only one fuse element is laid out, the laser beam L spread to below the fuse element has a risk of damaging various kinds of lower-layer wirings and the semiconductor substrate.